Over the last decade, minimally invasive surgery (MIS) has attained great popularity and acceptance among surgeons and patients alike. One example is laparoscopic surgery. However, MIS requires a high degree of competency from the surgeon due to the presence of a number of constraints, including limited vision. The surgeon is required to reconstruct the 3D operative field and perform instrument navigation through the narrow monoscopic two-dimensional (2D) field of view provided by the endoscope.
The perceptual cues that a surgeon may use to navigate the operative field are complex, and are often classified as primary (physiological) cues, such as binocular disparity, convergence and accommodation, and secondary (pictorial) cues, such as linear perspective, shading and shadow. The monoscopic field-of-view provided by an endoscope, for example a laparoscope, limits the 3D perception by projecting a scene onto a 2D plane. It has been observed that surgeons tend to compensate for the lack of depth perception by developing new strategies such as groping forward and backward with instruments to gauge the relative depths of organs by touching them. The combined visual and haptic feedback helps to confirm the instrument position and orientation.
This navigation approach, however, is not ideal particularly when undertaking delicate surgical manoeuvres that require subtle control of instruments which must be performed slowly and accurately enough to avoid damaging the tissues in contact.
Generally, there is always a constant requirement for surgery to become safer. This is particularly true in the current climate of clinical governance. Practically, safety can be achieved by better training as well as by reducing the constraints set by the nature of MIS. Improving 3D visualization and ultimately facilitating instrument navigation and manoeuvrings should hence be a priority.
Advances in true stereoscopic surgery aim to improve 3D perception, but such systems have practical limitations with respect to their use as they tend to be extremely expensive and not widely available. More importantly it has been demonstrated that these systems do not significantly improve endoscopic task performance, see for example R K Mishra, G B Hanna, S I Brown, A Cuschieri; Optimum Shadow-Casting Illumination for Endoscopic Task Performance; Arch. Surg 139: p889, Aug. 2004.
One of the primary cues that visual systems utilize to infer depth is shadow. Shadow can provide useful information about object shapes, relative 3D position, and surface characteristics within a scene. However, this visual cue is unavailable with MIS due to the coaxial alignment of the lens and the light source of traditional endoscopes. Under this setup, the operative field is generally shadowless. It has been shown in other studies that inducing shadow by the introduction of a secondary light source within the surgical field improves endoscopic task performance. See for example Mishra et al, cited above.
A first known prior art arrangement for introducing shadows in the operative field is disclosed in Mislra et al cited above. The operative field is accessed by an endoscope through a first cannula and by a surgical tool through a second canlula. A secondary light source is introduced through a third cannula placed above the tip of the surgical instrument or to the side of it. Each light source is fed by an independent halogen lamp. When testing the set-up on surgical trainees using an artificial surgical task, the shortest execution time and the least number of errors was found for a shadow cast by the overhead secondary light source producing a dark shadow. However, such a dark shadow also significantly obscures the operative field covered by the shadow.
An alternative set up for producing shadows in an endoscopic operative field is disclosed in M G Schurr, G Buess, W Klunert, E Flemming, H Hermeking & T Gumb; Human Sense of Vision: A guide to future endoscopic imaging systems; Min Invas Ther & Allied Technol 5: 410-418, 1996. The set-up employs a sideways looking endoscope with a primary light source illuminating the surgical field at an angle to the axis of the endoscope (and along its optic axis to the side) and a secondary light source provided by an illumination cannula with light bundles integrated into the shaft. The secondary light source provides a diffuse illumination which is off-axis with respect to the optical axis of the endoscope and thus produces shadows in the operative field.
Both approaches described above are problematic because, as shown by Mishra et al, optimum task performance requires a dense shadow. In the study of Mishra et al. the optimal shadow required a secondary light source much stronger than the primary light source (25000 lux and 6250 lux, respectively).
This means that, in the set ups of Mishra et al. and Schurr et al, the shadow cast by the surgical tool will obscure part of the visual field. Moreover, high intensity lamps are required for both the primary and secondary light source.